Inorganic fullerene-like (IF) nanoparticles and nanotubes are attractive due to their unique crystallographic morphology and their interesting physical properties. In particular, disulfides of molybdenum and tungsten belong to a class of solid lubricants useful in vacuum, space and other applications where liquids are impractical to use. The fullerene-like nanoparticles can be used as superior solid lubricants, e. g. as additives to fluids and for self-lubrication.
MS2 (M=Mo, W) layered compounds can be synthesized by sulfidization of the respective trioxides in a reducing atmosphere at elevated temperatures. Hollow MoS2 and WS2 onion-like (inorganic fullerene-like, IF) nanoparticles were first observed in thin (−20nm) films, which were formed by sulfidization of the respective amorphous MO3 films in a reducing atmosphere at temperatures of about 850° C. (Tenne, R., Margulis, L., Genut, M., and Hodes, G., Nature 360, 444 (1992); Margulis, L., Salitra, G., Tenne, R., and Talianker, M., Nature 365, 113 (1993)). In a search for the synthesis of a pure IF phase, it was suggested to take oxide powder rather than a thin film as a precursor material. However, MoO3 powder evaporates at temperatures above 700° C., while WO3 does not sublime 1400° C. Therefore, at the relevant reaction temperatures (˜850° C.), the reduction/sulfidization reactions of MoO3 and WO3powders occur by the gas-phase reactions (GPR) and solid-gas reactions (SGR), respectively.
In the case of the GPR, the size and shape of the final reaction product depends only on the prevalent conditions in the reactor, since MoO3 evaporates as (MoO3)3-5 molecular clusters (Magneli, A., J. Inorg. Nucl. Chem. 2, 330 (1956)). The reactor, which was used in an earlier report by some of the inventors of the present invention, (Feldman, Y, Wasserman, E., Srolovitz, D. J., and Tenne, R., Science 267, 222 (1995)) was very simple, indeed. A mixture of the three gases (N2, H2, H2S) was made to flow through the MoO3 vapor zone. IF-MoS2 powder accrued on the reactor walls along with a large number of other oxide and sulfide phases obtained by the GPR, when the MoO3 powder with crystallites of ˜5 μm was used as a precursor. To regulate the GPR process and increase the production yield of IF-MoS2, it was decided to separate the sublimation of MoO3 powder from its sulfidization reaction. Several modifications of the reactor enabled the preparation a few milligrams of an almost pure IF phase in a single run (Feldman, Y, Wasserman, E., Srolovitz, D. J., and Tenne, R., Science 267, 222 (1995)) . By varying the annealing time, the intermediate products of the reaction could be identified.
It should be understood that, according to the known mechanism for the growth of the IF phase in molybdenum or tungsten systems, the analysis of the kinetics of the simultaneous reduction and sulfidization of WO3 powders are based on the occurrence of a unique driving force for the fast growth of the first curved sulfide layer (0001) around an oxide nanoparticle. According to that kinetic model, a synergy between the reduction and sulfidization processes, which occurs in a very narrow window of parameters, leads to the formation of the first closed sulfide layers (Feldman, Y, Lyakhovitskaya, V., and Tenne, R., J. Am. Chem. Soc. 120, 4176 (1998)). On the basis of this model, the inventors have estimated the temperature regimes for the formation of small (10-30 nm) or large (100-200 nm) IF of WS2, WSe2, and WTe2 material, and experimental results have shown a good agreement with the predictions of the “synergy” model [Feldman, Y., Lyakhovitskaya, V., and Tenne, R., J. Am. Chem. Soc. 120, 4176 (1998); Tenne, R., Homyonfer, M, and Feldman, Y., Adv. in Metal and Semiconductor clusters, Ed M. A. Duncan, JAI Press Inc. 4, 227 (1997)]. The general principles of the sulfidization of the respective oxides, of the growth model were successfully applied also to the growth of IF in other layered systems, like VS2, In2S3, and SnS2 [Tsirlina, T., Feldman, Y, Homyonfer, M., Sloan, J., Hutchison, J. L., and Tenne, R., Fullerene Science & Technology 6, 157 (1998); Homyonfer, M., Alperson, B., Rosenberg, Yu., Sapir, L., Cohen, S. R., Hodes, G., and Tenne, R., J. Am. Chem. Soc. 119, 2693 (1997)].
It is necessary to point out here that the method of IF (including the nanotubes) formation is a “chemical” one: i.e., a chemical reaction is essential for the grow of these nanoparticles. Following this early work by the inventors of the present invention, a few reports on the synthesis of MoS2 nano and microtubes by other “chemical” methods have appeared in the literature, recently [Remskar, M., Skraba, Z., Cleton, F., Sanjines, R., and Levy, F., Surf. Rev. Lett. 5, 423 (1998); Remskar, M., Skraba, Z., Regula, M., Ballif, C., Sanjines, R., and Levy, F., Adv. Mat 10, 246 (1998); Vollath, D., and Szabo, D. V., Mater. Lett. 35, 236 (1998); Zelenski, M., and Dorhout, P. K., J. Am. Chem. Soc. 120, 734 (1998)].
It should be emphasized that the synthesis of carbon fullerenes using “physical” methods, implies curvature of very small atomic sheets followed by the annihilation of the dangling bonds of the peripheral atoms, a process, which is induced by a high rate of energy dissipation. It was shown recently that IF-MoS2 could be obtained also by “physical” methods, such as e-beam irradiation (Jose-Yacaman, M., Lorez, H., Santiago, P., Galvan, D. H., Garzon, I. L., and Reyes, A., Appl. Phys. Lett. 69, 8, 1065 (1996), or laser ablation of regular MoS2 powder (Parilla, P. A., Dillon, A. C., Jones, K. M., Riker, G., Schulz, D. L., Ginley, D. S., and Heben, M. J., Nature 397, 114 (1999), or by short electrical pulses from the tip of a scanning tunneling microscope through amorphous MoS3 nanoparticles Homyonfer, M., Mastai Y., Hershfinkel, M., Volterra, V., Hutchison, J. L., and Tenne, R., J. Am. Chem. Soc. 118, 33, 7804 (1996). IF-NiCl2, including nanotubes were recently observed after heating of NiCl2 at 960° C. in a reducing atmosphere (Rosenfeld-Hacohen, Y., Grunbaum, E., Sloan, J., Hutchison, J. L., and Tenne, R., Nature 395, 336 (1998). However, the IF yield in these cases is very modest and can not be compared with the large amounts of the pure IF-WS2 (MoS2) phase obtained by “chemical” methods.
The first SGR reactor for the synthesis of macroscopic quantities of IF-WS2 was constructed in 1996 based on the principles of the above indicated reaction mechanism. To increase the amount of the reactant (oxide) and expose its entire surface to the g, a bundle of quartz tubes was placed inside the main quartz tube (40 mm diameter) and the oxide powder was dispersed in them, very loosely. The reactor was introduced into a horizontal furnace and the powder was placed in a constant temperature region (˜850° C.). A stream of H2/N2+H2S gases passed through every tube filled with the powder. It created rather equivalent conditions for the reaction of the entire oxide powder during SGR. Typically, about 0.4 gram of IF-WS2 could be obtained in a single batch (3 hr), with a conversion yield of almost 100%.
The precursor (oxide) powder and the reaction products were analyzed by X-ray pox%der diffraction (XRD) and transmission electron microscopy (TEM). The inventors recall that the size and shape of the precursor tungsten oxide nanoparticles predetermine the IF-WS2 dimensions in the SGR synthesis. The starting material for the synthesis of IF-WS2 was a WO3 powder (>99% pure), with almost spherical particles having sizes smaller than ca. 0.2 μm. Oxide powder having a larger particle size was converted mainly into 2H-WS2 phase. Moreover, 2H-WS2 platelets were predominantly obtained if the packing of the powder was too compact. This fact required a very thorough control of the IF quality after every batch.
It was hypothesized before, that the tribological properties of the IF nanoparticles are closely related to their structure [(Rapoport, L., Bilik, Yu., Feldman, Y., Homyonfer, M., Cohen, S. R., and Tenne, R., Nature 387, 791 (1997). See also, Nature 387, 761 (1997); and Rapoport L., Feldman, Y., Homyonfer, M., Cohen, H., Sloan, J., Hutchison, J. L., and Tenne, R., Wear 2229, 975 (1999)]. In general, spherical IF nanoparticles exhibited tribological properties superior to their 2H counterpart, while their performance rapidly deteriorated upon loss of spherical shape. Therefore, it is expected that the more spherical IF-WS2 nanoparticles would perform very well as solid lubricants. In addition, the larger IF should be a better lubricant in the case of the friction of two matting metal surfaces having higher surface roughness.